Presently, there are considerable efforts underway to develop sustainable sources of energy. Research into wind-powered generators, wave-powered energy systems, and bio-fuels has increased dramatically in the last few years. Solar energy has always been seen as one possible source of environmentally-friendly energy. As such, research and development of higher efficiency solar cells has increased recently.
Most silicon-based solar cells utilize a portion of the wavelength spectrum of an incident optical signal to generate photo current. Single junction solar cells produce electricity when illuminated with light within specific, narrow wavelength range. In an effort to increase the efficiency, multiple junction solar devices have been developed. Unlike single junction devices, multi-junction devices feature multiple layers or junctions of photovoltaic/photoactive material. Each layer is configured to generate an electrical charge when irradiated with optical radiation within a specific wavelength range. Typically, the various layers are photoactive in different wavelength ranges, thereby providing a higher efficiency device than single junction solar devices.
Generally, it is desirable to characterize the spectral performance of a solar cell, during the research, development and fabrication phases of the solar device. The parameters currently used to characterize the spectral behavior of a photovoltaic device (PVD) are external quantum efficiency (QE) and internal quantum efficiency (IQE). To that end, PVD I-V curves are commonly used to characterize the global behavior of PVDs. From these curves the following parameters are obtained: Isc (short circuit current), Voc (open circuit voltage), maximum power, solar cell efficiency, and parasitic resistances. Presently there are several methods used to determine the QE of a solar device, such as the dual beam splitter method, integrating sphere method, and the fiber optic-based approaches. While each of the approaches has proven somewhat successful in the past, a number of shortcomings have been identified. For example, both the dual beam splitter method and integrating sphere method require either the sample under examination to be moved from one test station to another or one or more components within the test station need to be removed or replaced with different components in order to obtain reflectance measurements required for the measurement of internal quantum efficiency. As such, characterization of PVDs using these techniques tends to be a time consuming process. In contrast, the fiber optic-based approach offers a flexible testing platform without requiring the additional steps associated with the dual beam splitter approach and integrating sphere approach. Unfortunately, losses associated with light propagation through the fiber introduce uncertainties which could adversely affect the precision in the determination of the internal quantum efficiency.
Thus, in light of the foregoing, there is an ongoing need for improved quantum efficiency measuring systems capable of quickly and accurately measuring the quantum efficiency of samples under examination.